Electrostatic precipitators can be used for collection and thus removal of particulate from a gas stream in industrial processes. The density of particles in the gas stream can be reduced significantly by charging the particles by, via the discharge electrode of the electrostatic precipitator, generating charge carriers to become attached to the particles in the gas stream, and by applying a high voltage field so that the charged particles are forced towards the positive anode of the electrostatic precipitator, thereby removing the charged particles from the gas stream. The collected particles form a dust layer on the anode of the electrostatic precipitator, which is removed periodically by means of mechanical rapping devices.
The performance of an electrostatic precipitator energized can be impaired when treating high resistivity dust particles. The high resistivity dust causes a high electric field on the dust layer of collected particles in the electrostatic precipitator, which in turn can cause the electrical break-down of the dust layer, a phenomenon known as ‘back-corona’ or ‘back-ionization’.
Back-corona means that positive ions are generated by the breakdown of the dust layer, which neutralizes the beneficial negative ions generated by the discharge electrodes, which are used for charging the dust particles negatively. The result is a decreased voltage applied to the electrostatic precipitator and re-entrainment of the dust particles back to the gas stream due to small eruptions on the dust layer.
In present electrostatic precipitators being pulse energized, typically a smooth DC voltage with superimposed high voltage pulses of short duration is applied to the electrostatic precipitators. The pulse width typically lies in the order of or above 100 μs repeated at a certain frequency in the range of 1 to 400 pulses/s. The average current can be controlled by varying the pulse repetition frequency of a switching device in the system, while maintaining the voltage level applied to the electrostatic precipitator. In this way it is possible to limit or eliminate the generation of back corona and its negative effects to a large extent. It should be noted that the storage capacitor, the switching device and the inductance constitutes a series resonant circuit.
Two main architectures of pulse systems for precipitators exist: one based on switching at low potential and one based on switching at high potential. The first type normally comprises a pulse transformer and the switching takes place on the primary side as explained in U.S. Pat. Nos. 4,052,177, 4,600,411 and EP 0 108 963. EP 1 293 253 A2 is an example of the second type, wherein the switching takes place at a high potential.
U.S. Pat. No. 4,600,411 describes a pulse system with a transformer with a primary and a secondary winding and a thyristor switch. A power supply is connected to a charging inductor in series with a charging capacitor and a surge inductor connected to the primary winding of the transformer. A clamping network comprising a clamping diode and a parallel combination of a resistor and a capacitor is connected between the junction of the surge inductor and the charging capacitor for limiting the voltage across the surge inductor and the primary winding of the transformer.
U.S. Pat. No. 4,854,948 describes another pulse system with a transformer with a primary and secondary winding, a power supply connected to a storage capacitor and a thyristor circuit connected to the primary winding of the transformer. A diode connected to a parallel-connection of a capacitor and a resistor constitute a circuit for protection of the thyristor circuit. A voltage source supplies a base voltage of e.g. 35 kV to an electrostatic dust separator coupled to the secondary winding of the transformer. A detector is coupled to the dust separator for detecting rapid voltage variations that will occur in the event of sparks in the dust separator and for enabling the thyristor circuit to become conducting, thereby protecting the thyristor circuit. However, this detector is increasing the cost of the pulse system.
It is desirable to enhance the efficiency of the systems described in U.S. Pat. No. 4,600,411 and U.S. Pat. No. 4,854,948. Moreover, neither U.S. Pat. No. 4,600,411 nor U.S. Pat. No. 4,854,948 addresses the problem that the core of the transformer becomes saturated upon sparks inside the electrostatic precipitator, which aggravates the operation of the electrostatic precipitator substantially. Finally, the switching devices in the known systems are subject to potentially damaging high rates of di/dt in the case of sparks taking place in the electrostatic precipitator, hereby shortening the life times thereof.